1. Field of the Invention
The present invention relates to a flow passage switching valve for switching a flow passage of a fluid and, more particularly, to a flow passage switching valve provided with a valve element of a swing arm type.
2. Description of Related Art
For diesel engines, an exhaust gas recirculation (EGR) system has been adopted heretofore to reduce NOx in exhaust gas. When high-temperature exhaust gas is directly recirculated to an intake side of this EGR system, the high-temperature, expanded exhaust gas is supplied to an intake manifold. This increases the ratio of exhaust gas in a cylinder. Accordingly, the amount of air in the cylinder decreases, leading to lowering of combustion efficiency and also deterioration of exhaust gas components such as NOx.
To avoid the above problems, an EGR-cooler-attached EGR system has been developed. This system is structured such that an EGR cooler for cooling exhaust gas (EGR gas) by heat exchange with cooling water is disposed in part of an EGR passage to cool high-temperature exhaust gas (EGR gas), and the thus cooled gas is recirculated to an intake manifold. When the temperature of cooling water is low at engine start or at cold temperatures, on the other hand, exhaust gas (EGR gas) is likely to be excessively cooled, which causes lowering of combustion efficiency in a cylinder and deterioration of exhaust gas components. Accordingly, at engine start or cold temperatures at which the temperature of cooling water is lower than usual, the above EGR-cooler-attached EGR system is structured to allow exhaust gas (EGR gas) to flow in a bypass passage connected thereto to bypass the passage of the EGR cooler.
Further, a flow passage switching valve has been used to switch the flow of exhaust gas from one direction to either one of two directions or from two directions to one direction in order to switch the EGR cooler between use or nonuse. As such flow passage switching valve, many valves using butterfly valve elements have been put to practical use.
This flow switching valve using the butterfly valve element is however difficult to provide high dimensional accuracy, which may cause accumulation of deposits, leading to fixation of the valve element or interference to movement of the valve element to a full closed position.
Under the circumstances, using a flow passage switching valve provided with a valve element of a swing arm type in the EGR system has been proposed. One of the flow passage switching valves using such swing arm type valve element is a three-way switching valve with a rotatable cantilevered valve shaft and a flat valve seat to cause a valve element to come into surface contact with the valve seat, thereby improving the closability of fluid.
If used in the EGR-cooler-attached EGR system, however, even the above flow passage switching valve using the swing arm type valve element needs a pipe for EGR cooler and a bypass pipe to bypass the EGR cooler. This makes it difficult to reduce a mounting space of the system in an engine room.
Further, the above flow passage switching valve using the swing arm type valve element is likely to cause stagnation of exhaust gas around a support shaft (a bearing) due to a positional relationship between a path in each direction and the support shaft (the bearing). This also likely causes accumulation of deposits or stagnation of condensed water around the support shaft (the bearing). There is also a problem that the accumulation of deposits and stagnation of condensed water may increase resistance of operation of the valve element.